LTE (Long Term Evolution) uses OFDM (Orthogonal Frequency Division Multiplexing) in the downlink and DFT-spread OFDM (Discrete Fourier Transform spread Orthogonal Frequency Division Multiplexing) in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier (e.g., a frequency interval Δf) during one OFDM symbol interval.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms (#0 through #9) as seen in FIG. 2.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, as illustrated in FIG. 3. A resource block corresponds to one slot (0.5 ms) in the time domain (illustrated as the horizontal axis of FIG. 3) and 12 contiguous subcarriers in the frequency domain (illustrated as the vertical axis in FIG. 3). Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe (or transmission time interval, TTI) the base station transmits control information (illustrated as the Control region in FIG. 3) about to which terminals data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. The example shown in FIG. 3 includes a downlink system with 3 OFDM symbols as control.
To transmit data in the uplink the mobile terminal has to be assigned an uplink resource for data transmission on the Physical Uplink Shared Channel (PUSCH). In contrast to a data assignment in the downlink, assignments in the uplink must always be consecutive in frequency, to retain the single carrier property of the uplink as illustrated in FIG. 4. In the example shown in FIG. 4, a consecutive range of subcarriers (illustrated as the horizontal axis) is assigned User #1 for one subframe (illustrated as the vertical axis). User #2 has been assigned a larger range of consecutive subcarriers in the same subframe.
The middle SC-FDMA (Single carrier Frequency Division Multiplexing) symbol in each slot is used to transmit a reference signal (illustrated as the darkened blocks in FIG. 4). If the mobile terminal has been assigned an uplink resource for data transmission (illustrated as the clear blocks in FIG. 4) and at the same time instance has control information to transmit, it will transmit the control information together with the data on PUSCH.
Several UEs may be transmitting in the same subframe and transmission from said several UEs may be received simultaneously by an eNB (e.g. a radio access network device or a radio network node). It is desirable to keep the UEs time aligned at the eNB receiver. This preserves orthogonality between users, so that an FFT (Fast Fourier Transform) can be performed over the entire bandwidth in the eNB which will separate the users in frequency domain. It will also reduce complexity in the eNB to use a single FFT for all users.
The Physical Uplink Control Channel (PUCCH) is used for transmitting control information in uplink. The structure of PUCCH is illustrated in FIG. 5. The change in frequency is represented by the vertical axis, while change in time is represented by the horizontal axis. The transmission from the user occupies one physical resource block (illustrated as the darkened block) in each of the two slots in a subframe. Frequency hopping (illustrated as the darkened arrow) is used between the slots to get diversity.
In each physical resource block used for PUCCH, several users may transmit simultaneously. Code multiplexing is used to keep the signals orthogonal within the cell.
If a user need to transmit both data and control information, PUCCH is not used and the control information is instead multiplexed into the data transmission on PUSCH. Otherwise the single carrier property of the uplink signal would not be maintained.
In order to preserve the orthogonality in UL (Uplink), as mentioned above, the UL transmissions from multiple UEs need to be time aligned at the eNB. Since UEs may be located at different distances from the eNB (see FIG. 6), the UEs will need to initiate their UL transmissions at different times, given by a timing advance value. A UE (User Equipment) far from the eNB, e.g., UE2, needs to start transmission earlier than a UE close to the eNB, e.g., UE1. This can for example be handled by time advance of the UL transmissions, a UE starts its UL transmission before a nominal time given by the timing of the DL (Downlink) signal received by the UE. This concept is illustrated in FIG. 7.
In the example provided by FIG. 7, the UL timing alignment is maintained by the eNB (shown as ‘eNodeB’ in the topmost timeline of FIG. 7) through timing alignment commands, specifying the timing advance value, to the UE based on measurements on UL transmissions from that UE.
Specifically, downlink data (illustrated as blocks with a downward arrow) will first be received by UE1 and later by UE2 since UE1 is located closer to eNodeB, as illustrated in FIG. 6.
Through timing alignment commands, the UE is instructed to start its UL transmissions earlier, the further the distance to the eNB. This applies to all UL transmissions except for random access preamble transmissions on PRACH (Physical Random Access Channel), i.e. including transmissions on both PUSCH and PUCCH.
Specifically, the uplink data (illustrated as blocks with an upward arrow) will be sent at an earlier time by UE2 than UE1 so that the data from both users may arrive at the eNodeB at the same time. Thus, the uplink data from both users (UE1 and UE2) received by the eNodeB is time aligned.
There is a strict relation between DL transmissions and the corresponding UL transmission. Examples of this are:                the timing between a DL-SCH (Downlink Shared Channel) transmission on PDSCH (Physical Downlink Shared Channel) to the HARQ (Hybrid ARQ) ACK/NACK feedback transmitted in UL (either on PUCCH or PUSCH);        the timing between an UL grant transmission on PDCCH (Physical Downlink Control Channel) to the UL-SCH (Uplink Shared Channel) transmission on PUSCH.        
By increasing the timing advance (TA) value for a UE, the UE processing time between the DL transmission and the corresponding UL transmission decreases. For this reason, an upper limit on the maximum timing advance has been defined by 3GPP in order to set a lower limit on the processing time available for a UE. For LTE, this value has been set to roughly 667 us which corresponds to a cell range of 100 km (note that the timing advance value, referred to as TA value, compensates for the round trip delay).
As described above, the maximum TA value allowed by 3GPP limits the range of an LTE cell to 100 km. UL transmissions from UEs at larger distance than 100 km, where the maximum TA value has been reached and exceeded, will therefore not be aligned at the receiver with the UL transmissions of other UEs. This results in that                the transmissions from the UE will cause interference towards other users since the UL orthogonality is lost;        separate FFT processing from the other users is required which significantly increases the FFT processing requirements in the eNB receiver.        
In WO0111907, there is disclosed a method of operating a time-division multiplexed wireless communications system. A first group of terminals at a first range from base station is instructed to time their transmissions to arrive at a base station in synchronism with a first series of frames. A second group of terminals at a second range is instructed to time their transmissions to arrive at the base station in synchronism with a second series of frames that is time-offset with respect to the first series of frames. Transmissions from the first and second groups of terminals are received at the base station in synchronism with the respective first and second series of frames. The transmissions from respective first and second groups may be received on respective separate carrier frequencies, or may be multiplexed on a common carrier frequency. In cases where the same carrier frequency is used by the first and second series of frames, the intra cell interference may increase.